This invention relates to an improved method of and apparatus for operating a pump turbine, and more particularly, a method in which the corrosion of elements of the pump turbine system caused by oxygen dissolved in water is effectively prevented.
It is known that in a pump turbine, compressed air is supplied into a runner chamber at the pumping-up starting time and the water level in the runner chamber is forced downwardly from the runner so as to start the turbine in air. After starting, when the rotating speed of the runner increases and reaches a rated speed, an exhaust valve is opened thereby exhausting the compressed air in the runner chamber. Simultaneously, the water level in the runner chamber increases and the runner chamber is filled with water to take place pump shut-off operation and after detecting the priming pressure in the runner chamber, the exhaust valve is closed.
Although it is of course necessary that the compressed air in the runner chamber should be completely exhausted at the time of completely closing the exhaust valve, actually, it is impossible to completely exhaust the air in the runner chamber because of the whirling flow caused by water or the shapes of the runner and the covers, and particularly, in a high-speed machine operating under a high head, the runner and the flow passage are constructed to be flat so that in an extreme case, it becomes impossible to exhaust the compressed air and therefore to start the pumping-up operation of the turbine. In such a case, a large quantity of the oxygen in the air remaining in the runner chamber will be dissolved in the water by the whirling flow caused by the rotation of the runner whereby the amount of the dissolved oxygen increases.
In the normal operation of a pump turbine, after the starting of the pump, guide vams are opened to start the pumping-up operation, so that water containing a large quantity of oxygen is discharged into a reservoir located above the pump turbine system through a penstock. Therefore, there is no problem of damaging the elements of the turbine system by, for example, corrosion caused by the dissolved oxygen.
However, the pump turbine is not always transferred to the pumping-up operation immediately after the disconnection of the generator from the power transmission system. In such case, the turbine is maintained in the inoperative state for an indefinite interval during which the surfaces of the runner, the upper and lower covers, the guide vanes, etc. may be corroded by the dissolved oxygen, which leads to the lowering of the running efficiency of the pump-water turbine.
In this connection, FIG. 1 shows a relationship between the concentration of oxygen dissolved in water and corrosion speed of the elements. As shown in FIG. 1, the corrosion speed increases linearly until the amount of the dissolved oxygen reaches nearly 1% by volume of water, but above nearly 1%, the corrosion speed decreases rapidly, but local corrosion begins to occur.